Campi Flegrei

Introduction

Campi Flegrei (Phlegrean
Fields) is a quaternary caldera located west of the city of Naples in an
area of regional extension. The erupted products range in composition from
K-basalts to alkali-trachyte, phonolite. The complex has been active since
at least 47000 a bp, and it is surrounded by three other quaternary volcanic
centers:

a) Ischia Island whose activity ranges between 132000
a bp to the present;

According to several authors, activity in Campi Flegrei
itself has been dominated by two eruptions that produced widespread ash-flow
deposits: the 'Campanian Ignimbrite' (CI)
at about 34000 a bp, and the smaller 'Neapolitan Yellow Tuff' ( NYT) about
12000 years ago.

The area and its eruption products have been intensely
studied since the XVIII century, and, as a consequence, many theories have
been postulated about their origin. Breislak (1798) identified the CI and
considered it to be the product of mud eruptions from centers scattered
across the Campanian plain.

Scacchi (1848) firts considered that the CI was erupted
from Campi Flegrei; later (1890) he changed his mind and approached the
view of Breislak. De Lorenzo (1904) subdivided the activity of Campi Flegrei
into three periods including the CI in the first, the NYT in the second,
and the products of the recent activity inside the caldera in the third.
Zambonini (1919) was the first to interpret the CI as a deposit related
with an ash-flow mechanism of deposition.

Rittman et al (1950) postulated the existence of a central
volcano (the Archiflegreo) which was destroyed by the eruption of the CI
and the following caldera collapse; according to Rittman et al. the subsequent
activity produced several other collapses of lesser extent.

Rosi et al (1983), and Rosi and Sbrana (1987) have argued
for the existence of a ring fracture surrounding Campi Flegrei, and interpret
the pipernoid tuff of Camaldoli and some breccia deposits called Museum
Breccia (Johnston-Lavis, 1889 ) as the proximal deposits of the Campanian
Ignimbrite. They agree with Rittmann about the formation and limits of
a caldera in Campi Flegrei. Other authors (Di Girolamo, 1970, Barberi et
al, 1978, Di Girolamo et al, 1984, Lirer et al, 1987) believe that the
Campanian Ignimbrite was fed through an arcuate fracture on the northern
edge of Campi Flegrei; this eruption resulted in the collapse of a large
area including Campi Flegrei and part of the gulf of Naples.

According to Lirer et al (1987), the Campi Flegrei caldera
was mainly formed after another voluminous eruption which produced another
major pyroclastic deposit, the Neapolitan Yellow Tuff. Scandone et al (1991)
suggest that the Campanian Ignimbrite was fed through a NE-SW fracture
bordering the present Posillipo Hill and continuing into the Acerra depression.
These authors share Lirer et al (1987) view that the Campi Flegrei caldera
was formed after the NYT eruption.

The Campanian
Ignimbrite

The Campanian Ignimbrite (CI) has an average composition
of a trachyte . Di Girolamo (1970) identified some spatial variation in
the chemical composition of scoria and pumice suggesting an eruption from
a zoned magma chamber with the emission of progressively more mafic products.

According to Di Girolamo (1970), the mafic products cover
areas of lesser extent than the more acid members, suggesting a decrease
of explosivity in the course of the eruption. A detailed geological map
of the Campanian Ignimbrite has been made by Di Girolamo (1968); isopleth
distributions of lithics and pumice have been reconstructed by Barberi
et al (1978). Scandone et al (1991) have drawn new isopachs
based on the interpretation of data of drillholes in the Campanian plane

Both Di Girolamo (1968) and Barberi et al (1978) consider
the CI to be the result of a unique eruption, even if lateral facies variations
are observed. The more striking lateral variation is a change in color
from a poorly welded grey deposit to a more welded yellow one. This change
is explained in terms of a secondary mineralization and welding produced
by zeolitizatio n (Di Girolamo, 1968). On average the deposit is made up
of pumice and black scoriae, with a different degree of flattening, embedded
in an ashy matrix with subordinate lithics and crystals. Columnar jointing
and fumarolic pipes are often observed .

Di Girolamo (1968) identifies also a gradual vertical
facies variation: the lower part of the deposit is made up of a more welded
ash matrix with iso-oriented, collapsed black scoriae, a relative high
density and high unconfined compressive strength; toward the upper part,
the deposit is more incoherent, the scoriae have progressively lesser flattening
and are chaotically dispersed in the matrix. Di Girolamo (1968) uses different
terms to characterize the different degree of welding and density; they
are from the bottom to the top: piperno, pipernoid tuff, tuff, "cinerazzo"
(a local name for loose ash).

Outcrops, where the base of the CI is visible, are found
along the margin of the Campanian Plain. The occurrence of a pumice-fall
deposit at the base of the CI is observed only on the eastern side of the
Campanian Plain. About 80 % (volume) of the pumice-fall deposit is made
up of angular pumices with the remaining 20% of lithics (predominant) and
crystals. The pumices have a light grey colour, sometimes grading to reddish
in the upper part. Crystals are mostly sanidine with accessory biotite
and piroxene. An increase in the ratio lithics/pumice is observed in the
upper part of the deposit. An ash layer (~70 vol.% coarse-ash) of variable
thickness (~10- ~100 cm) is sometimes found above the pumice deposit. It
is made up by up to 90 vol.% of pumice and glass shards, and about 10 vol%
of lithic fragments and crystals (sanidine, biotite, piroxene, and, occasionally,
magnetite).

The initial phase of the eruption of the Campanian Ignimbrite
was characterized by a sustained eruption column which deposited a pumice-fall
deposit to the east of the source area, thus confirming the suggestions
made by Thunell et al (1979) ans Sparks and Huang (1980).

The Campanian Ignimbrite crops out along the border and
in dwelling and quarries of the Campanian Plain . The middle of the plain
is covered by the products of the recent activity of Campi Flegrei and
Vesuvius, and by alluvial terrains. As mentioned in the introduction, Rosi
et al (1983) and Rosi and Sbrana (1987) suggest that the lithic breccia
and the Piperno found in Campi Flegrei are lateral facies of the Campanian
Ignimbrite. Lirer et al (1991), Perrotta et al (1993) suggest that these
deposits have been produced by later eruptions.

The
activity after the eruption of the Campanian Ignimbrite

After the eruption of the CI (wherever it occurred) the
volcanic activity in the Campanian plain started to localyze on one side
at Vesuvius volcano, and onto the other side over a large area extending
from the area where presently there is the town of Naples as far as Procida
Island.

Procida and Monte di Procida

The island of Procida (fig) is
only a kilometer away from Monte di Procida, the western edge of Campi
Flegrei. The erupted products of Procida and Monte
di Procida range in composition from K-basalts to alkali-trachyte.

The age of the eruptions that occurred within this sector
span between 40 and 14 ka corresponding to the products of Vivara and Torre
Gaveta volcanoes. The activity covers a relatively long period before and
after the eruption of the Campanian Ignimbrite. Scandone et al (1991) tentatively
identified the products of the CI as the "S.
Martino pyroclastic flow". Rosi et al (1983,1987) identify the
CI as the products of the Museum Breccia.

The activity of this sector is mostly characterized by
explosive eruptions of limited energy as the products are not deposited
over large distances (at most a few kilometers). Pyroclastic flow deposits
are strongly controlled by topographic highs and often the products pinch
out on reliefs of a few tens of meters.

The oldest volcano is the islet of Vivara
which raises above sea-level as an isolated cone. Two lava domes (Punta
Ottimo, and S. Martino) were emplaced in the oldest period along with a
partially dismantled scoria cone (Miliscola). The areal extent of this
scoria cone must have been conspicuous as evidenced by a positive gravity
anomaly coincident with it (Cassano and La Torre, 1987). After these eruptions,
there was the deposition of several pumice layer of uncertain origin (probably
from Ischia). The Fiumicello eruption occurred at about 31000 y BP; the
center is probably on Procida Island where are found the most thick deposits
with a surge facies. The deposits of this eruption are found on Monte di
Procida as sequence of alternating grey ashes
and black lapilli. Th flow unit is characterized by black scoriae embedded
in an grey pumiceous and ashy matrix; it is strongly controlled by topographic
highs and in places (Torregaveta) has been strongly eroded by the overlaying
Museum Breccia made up by lithic fragments
and pumice lapilli (see detail).

The last eruptions on Procida were that of the Museum
Breccia (~18000 a BP according to Perrotta and Scarpati, 1993) and
Solchiaro (~17000 a BP) which erupted also
the less differentiated products (K-basalt). On the mainland, the last
eruption before the Neapolitan Yellow Tuff event was that of Torre
Gaveta.

Campi Flegrei

A schematic geological map of Campi
Flegrei is given in figure. On the eastern edge of Campi Flegrei, on Camaldoli
hill, the products of the activity of Campi
Flegrei are exposed. The oldest products are the so-called Torre-Franco
tuffs that comprise a series, 50 m thick, comprising ash-beds with cross-laminations
that alternate with pumice and scoria deposits interbedded with paleosols.

Above another paleosol is the Piperno-Museum Breccia formation.
The Piperno is a welded ash with abundant fiamme and some pipe-structures
that pass into the overlying breccia. As mentioned before Rosi et al (1983,1987)
identify the Piperno -Breccia Museum as the proximal facies of the CI and
suggest that the eruption of the Campanian Ignimbrite was responsible of
the formation of the Campi Flegrei caldera. According to these authors,
after the CI the area was invaded by the sea.

A subsequent activity occurred and filled the caldera
depression until the eruption of the Neapolitan Yellow Tuff. On the eastern
side of the caldera, above the Piperno there is a &ap;70 m thick sequence
of Whitish Tuffs (16 ka) overtopped by the Neapolitan Yellow Tuff.

The eruption of the Neapolitan
Yellow Tuff (NYT) occurred at ~12000 y BP. Rittman (1950) suggested
that the extensive deposits of yellow tuff that crops out around Campi
Flegrei and in the town of Naples were the results of different eruptions.
Lirer and Munno (1976) and Di Girolamo et al (1984) proposed that all the
deposits of yellow tuff outcropping outside and on the rim of Campi Flegrei
were the results of a unique eruption that produced the collapse of the
Campi Flegrei caldera (Lirer et al, 1987).

Rosi et al (1983), and Rosi and Sbrana (1987) followed
the views of Rittman (1950) and suggested that different eruptions were
responsible of the deposition of the Neapolitan Yellow Tuff. Recently,
Scarpati and Cole (1993) suggested the uniqueness of the Neapolitan Yellow
Tuff deposit based on a detailed stratigraphic, granulometric and geochemical
study of the different outcrops.

Scarpati and Cole (1993) suggest that the eruption of
the Neapolitan Yellow Tuff was firstly characterized by the deposition
of a phreato-plinian deposit of alternating pumice and ashes, followed
by the deposition of a huge sequence of surge and pyroclastic flows. The
deposits of the NYT are extensively found on the rim of the caldera and
within it, as evidenced by geothermal and water-well drill-holes. A conservative
estimate of its volume is between 10 and 20 km^3 of DRE.

After the NYT eruption, every activity outside the caldera
ended (Procida, Monte di Procida, Naples) and the following activity emplaced
only within the caldera structure, frequently along its rims and sometimes
involving intra-calderic collapses (Lirer et al, 1987).

Scandone et al (1991) suggest that after the eruption
of the NYT the inner part of Campi Flegrei was invaded by the sea and all
the subsequent eruptions occurred along the border of the caldera. The
major volcanic edifices built during this period have been eroded on the
side facing the sea.

Several major eruptions occurred between 11 and 9 ka abp.
These were the eruptions of Gauro (10 ka
bp), Archiaverno and of Agnano Pumices (9 ka), Monte Ruscello and probably
the volcanoes along the northern margin of the caldera (Montagna Spaccata,
Pisani, etc.) and Nisida, on the eastern
flank.

All these eruptions occurred along the rim of the caldera
and many of these volcanoes are made up by litified yellow tuff that has
been interpreted as an indication of water-magma interaction (Di Girolamo
et al, 1984). Other eruptions occurred along the western flank of the caldera,
and they were Baia and Fondi di Baia,
Miseno and Porto Miseno

An uplift of the caldera floor occurred between 10 and
5 ka bp (Cinque et al, 1985). A marine terrace (La Starza) raised to an
height of about 40 m presently borders the northern shore of the gulf of
Pozzuoli. The uplift was accompanied and followed by renewed volcanic activity
whose centers are slightly shifted toward the center of the caldera.

Local collapse during this phase resulted in the Agnano
caldera which was formed by several eruptions. Other major eruptions whose
products are well exposed, occurred at Cigliano, Agnano-Monte Spina (4000
abp), Astroni (3700 abp) and Averno (3700 abp),
Solfatara and Monte Olibano . The last
eruption in the area occurred in historical time and was that of Monte
Nuovo (1538 AD).

The recent unrest

Ground deformation

Since 1800, sea-level measurements made in the ruins of
a roman market (Serapeo) have indicated a slow
sinking of the area. (These slow movements of the ground have been locally
called "Bradisismo" by the greek bradi=slow, seism=movement).
Already in roman times there was evidence of sinking of the ground. The
Serapeo, built near the sea-shore, had its
floor elevated, two centuries after its first construction, because of
the invasion of the sea.

Levellings made at the beginning of this century showed
that the maximum sinking was occurring in the city of Pozzuoli and regularly
decreased eastward and westward along the coast. This slow sinking of the
ground continued until 1968. In the periods 1970-1972 and 1982-1984 two
important episodes of inflation occurred in the Pozzuoli area (Berrino
et al, 1984).

These episodes produced 170 cm (inferred with respect
to the previous levelling) and 182 cm, respectively, of uplift at the point
of maximum deformation (located in Pozzuoli). The inflation geometry is
the mirror image of the slow sinking observed until 1968; it has a circular
simmetry around Pozzuoli and regularly decreases toward the margin of the
caldera.

The inflation which occurred in 1970-72 had a partial
recovery of some 20 cm; similarly, a deflation of about 70 cm has occurred
since the end of 1984. One particular feature is the constancy of the areal
extent of the deformation.

Repeated levellings
(Berrino et al, 1984) showed that the bell-shaped form of the deformation
did not change appreciably during 1982-1984: altough it displayed a marked
vertical variation, its horizontal extent remained the same.

This pattern is not compatible with magma migration toward
shallow depth, for which a decrease of the areal extent of the inflation
would be expected. Scandone et al (1991) suggest that the deformation is
strongly controlled by the caldera structure and this explains the constant
planimetric geometry for inflation and deflation episodes.

The slow sinking of the ground observed until 1968 is
possibly related with the compaction of the loose pyroclastics that make
up the caldera floor. In this sense it represents the normal dynamics of
the area. On the contrary the sharp inflation episodes must be considered
as anomalous events related with a pressure increase either below the caldera
structure or within the loose pyroclastics.

Seismicity

A seismic crisis began in 1983, some months after the
beginning of an inflation of the ground. Earthquakes
occurred mostly in the coastal region around Pozzuoli, only a few, deeper
events occured within the gulf; however they did not extended outside the
border of the Campi Flegrei caldera. Hypocenters were located between a
few hundred meters until ~5 km depth. The maximum observed magnitude was
4.0.

A distinctive areal difference was observed in the pattern
of seismicity. Earthquakes with the shallowest foci occurred mostly as
swarms and were located in an area west of Pozzuoli; earthquakes located
in the eastern area, however, had higher magnitudes, occurred as single
or double events, and generally had deeper hypocenters. The events occurring
in the gulf resembled those of the eastern area, but with generally lower
magnitudes.

Aster and Meyer (1988) made a tomographic study of the
crustal structure of the caldera by a simultaneous tridimensional inversion
of velocity and hypocenters of earthquakes. They found that the central
part of the Pozzuoli caldera has an anomalously high vp/vs ratio and low
vp and vs, indicating an incompetent highly fractured medium, saturated
with liquid water, and that areas of anomalously low vp/vs occur on the
borders of the caldera depression.

The relocations of the deepest earthquakes showed an invard-dipping
elliptical zone of hypocenters, interpreted as a ring fault. The hypocenters
of earthquakes occurring in the gulf of Pozzuoli have not been plotted
in figure because they fall far from the
trace of the structural profile; however they are generally deeper than
3 km and possibly lie on the downward continuation of the south-western
fault zone of the caldera so suggesting a funnel-shaped caldera.